There are many applications where an infra-red transparent window is positioned in the optical path of EO equipment to protect the equipment during use. Where such windows are used on military vehicles, they can give unwanted radar returns if they pass a substantial proportion of any incident RF transmission, and particularly in the microwave band, which is understood to refer to radiation in the waveband of from a few MHz—300 GHz. This places stringent design criteria on the window so that it transmits in the infra-red and preferably also the visible spectral bands but does not transmit microwaves. The term ‘window’ is used broadly herein to mean an element capable of transmitting radiation in the optical and/or infra-red wavebands, said window being with or without optical power, and so includes lenses as well as refractive and diffractive elements generally. The term ‘transparent’ is used to mean that the window transmits a usable amount of radiation at the mid value of the infra-red and/or optical wavebands.
In order to provide low RF/MICROWAVE transmission infra-red windows it is known to apply a metal mesh or grid to the surface of the window. However, the metal mesh may have the drawback that, where the window is exposed to the environment, to air, water or sand abrasion, the thickness of the metal mesh can deflect water droplets or sand particles so as to accelerate abrasion of the window.
In some current applications, zinc sulphide windows that exhibit low RF/MICROWAVE transmission are used in EO sensors. Current designs incorporate fine metal grids close to the surfaces of the zinc sulphide windows in order to reflect microwave radiation. The bulk zinc sulphide windows are manufactured by chemical vapour deposition and the grid is then produced by sputtering/chemical vapour deposition onto the surface of the window, so producing large areas is a problem. Such designs are not cost effective; it is difficult to ensure uniformity of the metal grids, and there is a high risk of environmental damage to the metal grids.
Encapsulation of the grids by overgrowing with ZnS may circumvent the problems of environmental degradation, but this also suffers from scale-up difficulties and from induced defects in the surface topography caused by the grid sitting proud of the window surface and delamination or weaknesses at the interfaces. This problem can therefore create the need for additional post-fabrication machining or polishing in order to flatten the outer surface of the grown on ZnS.